The human olfactory amygdala: Anatomical connections between the olfactory bulb and amygdala subregions

The human olfactory bulb is thought to send direct monosynaptic projections to the amygdala, though anatomical evidence for this is scant. Here, we applied a specialized diffusion-weighted magnetic resonance imaging protocol optimized for olfactory brain areas to systematically quantify connections between the olfactory bulb and the amygdala in 25 healthy human participants. We found that the olfactory bulb exhibits a higher density of streamline connections to the medial nucleus, the anterior cortical nucleus, the central nucleus, and the periamygdala complex compared to the basomedial nucleus, the basolateral nucleus, the lateral nucleus, and the posterior cortical nucleus. We used k-means clustering algorithms to confirm these results by performing a data-driven grouping of amygdala subregions into those that connect to the olfactory bulb and those that do not. We further found that olfactory amygdala subregions and non-olfactory amygdala subregions exhibit different structural connectivity patterns with the rest of the brain. Our findings provide confirmatory evidence that a set of amygdala subnuclei-medial nucleus, anterior cortical nucleus, central nucleus, and periamygdala complex-communicate with the olfactory bulb and contribute to primary olfactory processing in humans.